Neural stem cells generate the diverse neuronal and glial cell types in the brain. In the cerebral cortex, cortical projection neurons are sequentially generated from the radial glial cells. The deep-layer neurons are born first, followed by upper-layer neurons. However, the cellular mechanism that regulates the radial glial cells to generate these diverse cell types is not well understood and is controversial. Two different models have been proposed. Early cell transplantation experiments, viral lineage tracing and in vitro cell lineage analysis suggested a model that postulates that early radial glial cells (RGCs) are multipotent, and sequentially generate different neural cell types and glia based on birthdate. A new model has been proposed that postulates that the early RGCs contain intrinsically lineage restricted stem cells which in turn give rise to specific neuronal subtypes. We propose to test these different models and determine the cellular mechanisms underlying the generation of different projection neuron subtypes and glia during cortical development. We will also determine whether multipotent adult neural stem cells exist in healthy adult mammalian brains.